In the world of computers, the viewing angle is the direction from which a display -- whether a boxy monitor or flat-panel display -- will look the best. Many different guidelines have been published about the best viewing angles and distances, but in most cases, the guidelines ignore the proven link between viewing angle and viewing distance. Scientists who study the visual system have found that the distances allowed in these guidelines are usually too close and the angles too high. This can cause eyestrain and other workplace-related health problems.
The eye has a natural "comfort zone" when it comes to focusing for distance, called the resting point of accommodation. This varies from person to person, but on average it is about 31 inches. Looking too long at a monitor that is closer than this increases eyestrain. This is because the eye muscles must work twice as hard to focus on a monitor 12 inches away than for one that is 30 inches away. The closest distance at which an object can be brought into sharp focus is called the near point of accommodation. Again, this varies from person to person. And as we age, the lens in the eye changes so that near point gets farther away. That's why many people over 40 find they require reading glasses.
When we work on a computer, we are operating at near distances. Viewing a monitor or other close object causes our eyes to accommodate and converge. First, an eye muscle contracts to adjust the shape of the lens, bending the incoming light so that it strikes the back of the eye at a single, focused point. If that point is too far in front of, or behind, the back wall of the eye, things look blurry. The brain responds to the blurry image and instructs the muscle to alter the shape of the lens to bring the object into sharper focus.
The closer the object, the more the eye muscles tend to converge inward toward the nose. This is called convergence, and it allows both eyes to focus the object at the same place on the retina. The eyes also have a natural resting point of vergence, usually around 40 inches. Looking at objects closer than this can strain the muscles that control vergence. The closer the distance, the greater the strain.